1. Field of the Invention
The present invention relates generally to the field of valves, and more specifically, to a rising (or reciprocating) stem valve that incorporates a ball screw mechanism and prevents leakage offend to the atmosphere.
2. Description of the Related Art
Attempts have bean made to provide leak-free protection for rising stem valves, which include gate, globe, knife and needle valves. Currently, metal bellows are employed around rising stems in these valves, especially when the valves are handling hazardous fluids. The bellows surround the stems and their associated packings to contain any leaks that penetrate through the packing assembly. Bellows, however, are not inherently leak-free because they eventually tail as metal fatigue begins to form cracks in the bellows. This kind of failure may result in a catastrophic release of hazardous fluid because when the bellows fails, the packing leaks without restraint.
An alternative, inherently leak-free technology is needed to replace bellows in places like chemical plants, refineries, paint factories, and cryogenic applications, where rising stem valves are integral to the functioning of the plant itself. This alternative technology must provide the advantage ox completely containing any leakage of fluids from valves.
A number of patent applications have been filed for valve actuators that mitigate stem leakage through the use of a magnetic interlock. These actuator chambers either enclose the dynamic seal that is present in every valve around the stem of the valves or eliminate the need for the seal entirely. This dynamic seal is known as a packing or mechanical seal. The magnetic interlock is employed to transmit force from the outside to the inside of the actuator chamber, thus avoiding the penetration of the chamber wall by a mechanical stem actuator. Penetration of the chamber wall would nullify the purpose tor the chamber in the first place—to enclose the dynamic seal around the stem and prevent leakage from the seal.
The problem with various proposed magnetic actuators is that the amount of force transmitted by the magnets is not adequate to ensure the proper function of the valve. If an actuator is designed to provide adequate force to open and close the valve, the magnet coupling is so large as to make it impractical. Even with the use of modern rare-earth magnets such as Neodymium Iron Born and Samarium Cobalt, the ability to transmit adequate force to the valve stem is still difficult. The forces provided by the magnets are only a fraction (usually less than 20%) of the force that a mechanical stem actuator can provide. This does not give the valve operator the confidence that his valve can be opened or closed under situations where high force is required, such as high fluid pressure, dry seals, or debris in the fluid path.
Rather than increasing force by building ever larger magnetic couplings, the present invention incorporates a ball screw assembly that multiplies the force supplied by the inner magnetic coupling while at the same time converting that force from rotary to reciprocal motion. For example, a torque of 120 in-lbs. can be converted to an axial force of 2000 lbs. or more by using a typical 0.75″ ball screw with a lead of 0.5″, the lead being the distance that the screw moves axially with each full rotation of the ball nut. This allows the use of a much smaller magnetic coupling. The reduction in size is desirable because the magnetic coupling is the most expensive component of the actuator.
Through the incorporation of a ball screw subassembly, the present invention provides a magnetically activated valve actuator that can be used in the harshest conditions. Magnetic actuation is no longer appropriate for light applications only. Rather, it is a robust alternative that provides force to the stem that is equivalent to that of low- and medium-pressure dynamically sealed stemmed valves. This innovation is most needed in places like chemical plants, refineries, and pipelines where valves are the central workhorses of the plant or pipeline.
In addition to increasing force and/or decreasing the size of the magnetic coupling, the present invention has the advantage of completely containing any leakage of fluids from the valve bonnet. The present invention is intended to be coupled to valves that are used in hazardous fluid or chemical applications, where stem leakage poses a pollution threat to the outside environment or a safety threat to personnel working nearby. At the very least, leakage from stem packings results in the loss of product, which can be costly. Fugitive emissions account for over 125,000 metric tons of lost product per year in the United States alone. Of this amount, the percentage of fugitive emissions that come from valve stems is estimated to be between 60% and 85%.
The threat posed to the environment by leaking valve stems is great, particularly when the product that is leaked is a fugitive emission, that is, a leaked or spilled product that cannot be collected back from the environment. An example of a fugitive emission would be methane leaking from a valve on a pipeline or in a refinery, in which case the methane immediately goes into the atmosphere and cannot be recaptured. Another example would be crude oil leakage from a valve on an offshore rig, where the oil is earned away by ocean currents and cannot be recovered.
Safety requirements are becoming more stringent with each passing year. Personnel who are required to work near hazardous chemicals—such as operators in a petrochemical plant—are subject to injury from leaking valve stems, especially from reciprocating stems where the hazardous material inside the valve is transported to the outside environment via the stem as it retracts from the valve body. For example, if the valve is handling chlorine, a leaking stern transports it to the outside environment, where it becomes hydrochloric acid when it reacts with moisture in the air. This acid corrodes the stem, which makes it even more difficult to seal over time.
The magnetic actuator of the present invention safely encloses the stem of all reciprocating stemmed valves because it is able to transfer torque through the enclosure magnetically without physically penetrating the enclosure itself. Magnetic actuators have been proposed previously for rotating stem valves. For these devices, the torque is magnified inside of the actuator chamber by the use of a worm gear or a planetary gear set. In the case of rising stem valves, however, the torque must also be converted to reciprocal motion. The present invention proposes the use of a ball screw, which not only magnifies the force of the magnetic actuator, but also converts the rotary motion to reciprocal motion.
Currently, ball screws are being used to actuate high-pressure gate and globe valves where large forces are required to move the valve stem up and down. For example, in a high-pressure, ASME Class #2500 gate valve, the fluid pressure on one side of the gate may be in the range of 5,000 psi, pushing the gate against the downstream valve seat with several tons of force. To lift the gate, the stem must provide as much as 20,000 lbs. or more of lifting force.
Ball screws and helical spline actuators are employed in high-pressure, self-contained hydraulic, electric, or pneumatic actuators, where the actuation force is transferred into the sealed chamber or outer casing by means of electrical wires, hydraulic fittings, or pneumatic fittings. The hundreds or thousands of ft-lbs. of torque required to move these valve stems cannot be transferred magnetically in a practical way; therefore, it has not been obvious that magnetic couplings could ever be coupled to ball screws to actuate rising stem valves. Instead, the automated versions of these valves are self-contained; that is, the mechanical energy required to actuate these valves is provided internally by hydraulic, electric, or pneumatic means.
The present invention cannot be used tor the applications described above for the reasons stated; however, it can be used for low- and medium-pressure applications known as ASME class #150, #300, and #600 valves. The ball screw specified in the present invention is much smaller in diameter than those currently being used to actuate high-pressure valves. This accomplishes three things: (1) the lower torque requirement allows the use of magnetic actuation rather than self-contained power (that is, the transfer of torque through the sealed chamber by means of a magnetic coupling is now possible); (2) the smaller diameter ball screw allows for more room for the inner magnetic cartridge, making it possible to consider high-temperature Alnico magnets tor use at temperatures up to 950 degrees Fahrenheit; and (3) the mechanical advantage provided by the smaller diameter ball screw (i.e., the ratio of reciprocal force over supplied torque) is much greater than that of the larger high-pressure ball screws when given the same amount of axial travel (or lead) per rotation. (A larger diameter screw has a larger circumference per rotation, which results in a greater axial movement per rotation than with a smaller screw that has the same lead angle. Thus, the smaller screw must have a steeper lead angle in order to supply the same amount of axial travel per rotation as a larger diameter screw. A steeper lead angle increases the efficiency of the screw when converting rotary motion to reciprocal motion.) These advantages are not present in any of the prior art valves that utilize a magnetic actuator.
Examples of valve designs involving magnetic actuators include: U.S. Pat. No. 3,908,959 (Fichtner, 1975); U.S. Pat. No. 4,284,262 (Ruyak, 1981); U.S. Pat. No. 4,296,912 (Ruyak, 1981); U.S. Pat. No. 4,327,892 (Ruyak, 1982); U.S. Pat. No. 4,382,578 (Ruyak, 1983); U.S. Pat. No. 4,384,703 (Ruyak et al., 1983); U.S. Pat. No. 4,671,486 (Giannini, 1987); U.S. Pat. No. 5,039,061 (Heard et al., 1991); U.S. Pat. No. 5,129,619 (Castetter, 1992): U.S. Pat. No. 5,129,620 (Castetter, 1992); U.S. Pat. No. 5,372,351 (Oliver, 1994); U.S. Pat. No. 8,297,315 (Esveldt, 2012); U.S. Pat. No. 8,490,946 (Burgess et al., 2013); U.S. Pat. No. 8,496,228 (Burgess et al., 2013); and U.S. Pat. No. 8,690,119 (Burgess et al., 2014). An example of an attempt to solve the problem of providing a leak-proof valve for cryogenic applications is U.S. Pat. No. 5,356,112 (Simar et al., 1994). An example of a valve that converts rotary motion to linear (reciprocating) motion is U.S. Pat. No. 7,325,780 (Arai et al., 2008). An example of a gate valve that utilizes a motorized ball screw actuator is U.S. Patent Application Pub. No. 2011/0308619 (Martino et al.).